Many bicycles, including most BMX bicycles, employ bicycle crank sets that are comprised of three major structural components. These components include two “handed” crank arms, and one central, axial, connecting crank shaft, which is an axle and is also referred to as a spindle. To function properly, the assembled components of a bicycle pedal crank assembly must be torsionally stiff so that the relative orientation of the crank arms can be maintained to transmit all of the force applied into the pedaling drive. The joints between the components are usually expensive to produce and inherently add weight to the structure. Also, there are undesirable stress concentrations in most conventional bicycle pedal crank assembly designs.
Typical BMX bicycle pedal crank assemblies employ one of two different types of configurations. The first arrangement employs mating splines on the spindle and in corresponding sockets in the two crank arms. In this arrangement the sockets have an unbroken outer wall formed in the structure at the crank end of each of the crank arms. The other popular crank assembly configuration employs mating splines or mating flats on the spindle and arms, but with a radial opening defined in the wall surrounding each socket. The crank arms are provided with outer “pinch” clamping bolts that, when tightened, reduce the width of the gap at the radial openings in the socket walls.
The conventional spline system in which there is no radial gap in the socket wall has inherent problems. Specifically, the spline must have a good interference fit so that no “slop” or “wobble” of the crank arm relative to the spindle is possible during pedaling. With the correct interference fit, the oscillating direction of the torque applied during the pedal stroke cycles will not impose strains larger than those of the fit.
While this firm, structurally secure connection provides excellent force transmission characteristics and reduces stress in the bicycle pedal crank assembly components, disassembly of the crank assemblies is very difficult, even for experienced users. That is, the spline fit is so tight that it is extremely difficult to remove either crank arm from the spindle to repair or replace components parts of the bicycle pedal crank assembly, or of bicycle parts that are engaged by the assembly. If the “fit” of the spine is relaxed and the tolerance of fit between the external splines on the spindle and the internal splines on the crank arm sockets is increased, assembly and disassembly is easier. However, the increased tolerance in fit results in the cyclical pedal force producing wear upon both the sockets in the crank arms, and also the splines on the spindle. As a result, the entire assembly is loosened at regular intervals. Unwanted impact stresses are then produced on the spindle.
In the other popular conventional system in which pinch bolts are employed, the use of clamping bolts facilitates assembly and disassembly when the clamping bolts are loosened. Conversely, a very tight fit between the splines of the crank arm sockets and spindle ends can be achieved by tightening the pinch bolts. However, crank set designs that employ pinch bolts remove a substantial portion of the structural strength of material of each crank arm end surrounding the socket. This results from the radial split in the socket area of the crank arm that is already under high stress. “Pinch” bolt designs are also unpopular with many riders, both due to their physical appearance, and because of injuries that can result to the user while riding due to the additional mass at the coupling end of the crank arm necessary to house the pinch bolts.